Soil Microorganisms Alleviate the Allelochemical Effects of a Thyme Monoterpene on the Performance of an Associated Grass Species

Abstract

BackgroundPlant allelochemicals released into the soil can significantly impact the performance of associated plant species thereby affecting their competitive ability. Soil microbes can potentially affect the interaction between plant and plant chemicals by degrading the allelochemicals. However, most often plant-plant chemical interactions are studied using filter paper bioassays examining the pair-wise interaction between a plant and a plant chemical, not taking into account the potential role of soil microorganisms.Methodology/Principal findingsTo explore if the allelopathic effects on a grass by the common thyme monoterpene “carvacrol” are affected by soil microorganisms. Seedlings of the grass Agrostis capillaris originating from 3 different thyme sites were raised in the greenhouse. Seedlings were grown under four different soil treatments in a 2*2 fully factorial experiment. The monoterpene carvacrol was either added to standard greenhouse soil or left out, and soil was either sterilized (no soil microorganisms) or not (soil microorganisms present in soil). The presence of carvacrol in the soil strongly increased mortality of Agrostis plants, and this increase was highest on sterile soil. Plant biomass was reduced on soil amended with carvacrol, but only when the soil was also sterilized. Plants originating from sites where thyme produces essential oils containing mostly carvacrol had higher survival on soil treated with that monoterpene than plants originating from a site where thyme produced different types of terpenes, suggesting an adaptive response to the locally occurring terpene.Conclusions/SignificanceThe study shows that presence of soil microorganisms can alleviate the negative effect of a common thyme monoterpene on the performance of an associated plant species, emphasizing the role of soil microbes in modulating plant-plant chemical interactions.

An allelopathic plant facilitates species richness in the Mediterranean garrigue

“…The essential oil is sequestered in glandular trichomes on leaves and stems and enters the soil during litter decomposition of thyme leaves. When applied to sterile soil in concentrations similar to that found in natural situations, Ehlers (2011) found that survival of a grass was reduced by 80% relative to soil without the amendment of a thyme monoterpene. However, on non-sterile soil, this allelopathic effect was less severe, showing that the presence of soil micro-organisms can alleviate the negative impact of monoterpenes on associated plants.…”

“…These compounds are known for their allelopathic properties affecting the performance of associated species. They can inhibit germination, growth and survival of other plants (Aspelund 1968;Langenheim 1994;Tarayre et al 1995;Abrahim et al 2000;Ehlers 2011), growth of micro-organisms (e.g. Vokou et al 1984) and can act as a deterrent against herbivores .…”

Soil microbes alleviate allelopathy of invasive plants

“…It has been reported that allelopathic activity of carvacrol is stronger in sterilized soils than that in unsterilized soils [18]. Blum [19] found that soil microbes are able to degrade phenolic acid and then decrease its phytotoxicity and duration.…”

Biodegradation of the Allelopathic Chemical m-Tyrosine by Bacillus aquimaris SSC5 Involves the Homogentisate Central Pathway

“…Thus, soil microorganisms can either enhance or decrease the phytotoxic properties of allelochemicals which interferes with defense properties of m -tyrosine producing plants. In some cases, the microbial degradation products of the allelopathic chemicals pose potential phytotoxic effects, while some of the microbially transformed products of allelochemicals decrease their level of phytotoxic effect as compared to the original compounds [3], [18], [19], [20], [21], [22], [23], [24], [25]. Several studies reported microbial degradation of previously characterized allelochemicals such as sorgoleone, juglone, benzoxazinoids, and other flavonoids [26], [27], [28], [29], while there is no information available on the degradation of m -tyrosine by means of bacterial isolates.…”